1. The Field of the Invention
The present invention relates to methods and apparatus to improve four important aspects of cementing casing in a well bore. They are discussed in the order that the operations are generally performed in the field: (1) Improved placement of the cement during initial cementing of the casing in the well bore; (2) Preventing gas migration into the cement slurry after placement; (3) Improving the tests used to evaluate cement placement; and (4) Improving the success of remedial cement squeeze operations.
The basic concept of the present invention is to apply a random or periodic, pulsating, oscillating or vibrating pressure to the fluids present at different stages during the cementing operation. The effect is to reduce or eliminate gelation of the fluids.
The tendency of fluids in wells to develop gel strength under static conditions interferes with the cementing operation. One aspect of this invention is the discovery that if fluids are oscillated at the surface of the well, this motion is efficiently transmitted long distances down the well, where it prevents the development of gel strength, or reverses the process of gelation, if it has already occurred earlier in the operation.
2. The Prior Art
After a well has been drilled, casing is typically lowered into the well bore and is cemented in place by pumping a liquid cement slurry into the annular space between the casing and the well bore. This generally requires the displacement of a drilling fluid from the annulus by the liquid cement slurry. Drilling fluid tends to gel under the static conditions that exist just before the cement slurry is pumped downhole. When the cement slurry is pumped into the annular space between the casing and well bore, it may bypass pockets of gelled drilling fluid, leaving incompletely cemented casing.
Several techniques are routinely employed in the oil industry to improve displacement of the drilling fluid in the annulus by the cement slurry. They include thinning and circulating the drilling fluid prior to cementing, rotating or reciprocating the casing before and during cementing, the use of centralizers, etc. All of these methods add significantly to the expense and time required to cement the casing and they may not provide a proportioned improvement in the displacement of the drilling fluid by the cement slurry.
Casing vibration has been shown to improve cement displacement during large scale tests and it has been proposed as a method to prevent gas migration, which is discussed below. A device for commercial application has been constructed. It is a large hydraulically operated device that mechanically supports and vibrates the casing. It is very expensive and difficult to use in the field and has not proved to be a practical device. It has never been used for cementing operations because of its prohibitive cost, but it has had limited success in freeing stuck drill pipe when all other methods have failed.
The migration of gas from gas formations into the cement slurry may occur after the cement has been pumped, but before it has become a solid. This represents a common problem in some fields and may occur on an unpredictable basis in others. The consequences range from "gas cut cement" to blow outs to the surface. The control of gas migration is one of the most costly and challenging technical problems in well cementing.
The basic cause of gas migration is believed to be the loss of hydrostatic pressure within the cement column as it makes the transformation from a liquid slurry to a solid. The development of gel strength in the static column of cement is primarily responsible for this loss of hydrostatic pressure. This loss of hydrostatic pressure allows an influx of formation fluids, usually gas, before the cement has completed the setting process.
Various chemical additives have been tried to control gas migration in cement slurries. Some of these additives appear to be completely ineffective, while others appear to have different degrees of effectiveness. But all are very expensive and most are of limited applicability. Such additives typically increase the cost of cementing casing by a factor of two to five times.
Relatively few mechanical methods have been used to control gas migration and only one is in common use. It involves cementing a short column of cement across a gas zone known to be a problem and leaving a column of drilling fluid over the cement slurry to maintain hydrostatic pressure as the cement sets up. This technique may be used with a cement "staging tool" to complete the cementing operation. This method has enjoyed a degree of success, but it significantly increases the cost and time required.
Gas migration can be prevented if gelation of the cement slurry can be prevented or delayed until the cement slurry develops enough viscosity to prevent the movement of gas within the slurry. This can be accomplished by mechanical agitation. It has been reported that slowly rotating the casing, after the cement has been placed but before the slurry sets up, can prevent gas migration. Clearly this method is limited to applications where casing rotation is practical. Special equipment is required to accurately control the torque. Rotation must be stopped when the drag on the casing at the bottom of the well becomes too high and before torque builds to the point that the casing might be twisted off. This may occur before the cement is viscous enough the prevent gas migration at shallower depths. This is because cement slurries begin to thicken and set up at the bottom of wells first, because the temperature is higher.
Cement bond logs are the primary method used to evaluate the placement of cement between the casing and well bore. For the purpose of this discussion, the term cement "bond log" includes all devices that rely on acoustic logging devices used to evaluate the placement of cement. It also refers to the test results of such a device.
These cement bond logs heretofore have been subject to a number of shortcomings. A good bond log generally means that a good bond has formed between the cement and the pipe or casing, but a poor bond log does not necessarily indicate poor cement placement. There are many reasons why a poor bond log may be obtained, and none of them pertain to the actual cementing of the casing.
Advances are continually being made in the use of acoustic logs, but they all share one major shortcoming, namely they require a very good physical contact between the cement and the casing in order to conduct sound into the cement and formation. A gap of only a few thousandths of an inch between the cement and casing may be detected by the log as no cement at all between the casing and well bore. Any one of a number of processes, such as temperature cycling, cement shrinkage, casing contraction, etc., can cause gaps to form as the cement sets.
If a poor bond log is obtained, remedial cement squeeze operations are generally performed until a satisfactory bond log is obtained. Several squeeze operations may be required. A substantial body of field evidence indicates that these squeeze operations are often unnecessary and that the problem is the bond log rather than the overall quality of the cementing operation. That is to say, the cement bond log is taken to represent the extent of displacement of the drilling fluid by the cement slurry during cement placement and it is overly sensitive to factors that do not relate to the quality of the cementing operation itself.
Strategies have been developed to improve the ability of bond logs to accurately represent the completeness of the drilling fluid displacement by the cement. For example, it is now common practice to run bond logs with the casing pressurized. The expansion of the casing, when pressurized at the surface to pressures of the order of 1,000 to 2,000 psi, may close any "microannulus" that formed during cementing thereby providing an improved bond log.
However, there are also instances where pressurizing the casing does not close this gap. Excess pressure can enlarge a microannulus and/or form cracks in the cement sheath that are detrimental to the seal provided thereby. Expansive cement additives have been developed with the goal to improve bond logs. They have had a more limited application than pressurizing the casing and they may actually reduce the strength of the cement. All of these methods are time consuming and of uncertain reliability.
There are a variety of circumstances where it is desired to "squeeze" cement slurries into areas of a wellbore that are already occupied by gelled fluids, usually drilling fluid. A common example is to repair casing leaks in uncemented sections of casing. In this example, gelled drilling fluid is usually left in the annulus between the casing and the well bore and the objective is to squeeze cement into this annulus to seal off the leak to prevent unwanted formation fluids from leaking into the casing.
During such a squeezing operation, the cement often channels or fingers through the gelled fluid rather than displacing it away from the point of entry in a uniform fashion. Once such a channel has formed, large quantities of slurry will tend to flow through the same channel without displacing the gelled drilling fluid surrounding the channel. The result is an unsuccessful squeeze operation.
Multiple squeezes may be required. This involves squeezing, waiting for the cement to set, drilling out the cement left in the casing, pressure testing, squeezing again, (this time through a different channel,since the first one is full of set cement) etc, until a successful pressure test is obtained. Two or three squeezes are often required for a successful pressure test. Cases have been reported where a dozen or more squeezes were necessary to obtain a successful pressure test.